Conventional organic light-emitting diodes (OLEDs) have a transparent substrate, typically indium-tin-oxide (ITO) glass, through which the light is emitted, and the surface electrode such as MgAg or LiF/Al is highly reflective. The electrode provides a surface from which internally generated light is reflected and directed toward the glass substrate, and thus improves the brightness of the emission. However, such an electrode also reflects ambient-light entering the device structure, thereby degrading the visually perceived contrast of the emitted light, as viewed by an observer. In numerous practical applications, it is of importance that an OLED can be easily viewed under ambient-light conditions ranging from total darkness to total sunshine so that a sufficient reduction is required in reflection of ambient-light from the mirror-like surface of the electrode. A well-known approach to reducing glare attributed to ambient-light is to use polarizers, particularly circular polarizers, which may be bonded to an outside surface of the light-transmissive substrate. However, the use of polarizers adds additional cost, and a polarizer bonded to a substrate is not a part of the integral layer structure of an OLED.
Hung and Madathil have disclosed an OLED with an absorbing layer formed of oxygen-deficient zinc oxide in U.S. Pat. No. 6,429,451. In such a case the zinc oxide layer was combined with an ultrathin bilayer of LiF/Al between the absorbing layer and the Alq layer to form an OLED with sufficient reduction in ambient-light-reflection from the mirror-like surface of the metal electrode through optical destructive interference. A. N. Krasnov disclosed that an absorbing layer of co-deposited Al and SiO combined with an ultrathin bilayer of LiF/Al between the absorbing layer and the Alq layer formed a reflection-less cathode (“High-contrast organic light-emitting diodes on flexible substrate” Alexey N. Krasnov, Appl. Phys. Lett. 80, 3853 (2002)). However, the deposition of oxygen-deficient zinc oxide by an electron beam is not compatible to OLED preparation, as the devices are extremely sensitive to radiation damage incurred in the deposition process. It is extremely difficult to control the composition of the absorbing layer formed of oxygen-deficient zinc oxide or mixed Al and SiO, and any deviation from an optimum composition would result in a significant change in its resistivity, refractive index & optical absorbance and thus considerably affect optical interference. Moreover the reflectance in the prior art is reported to be less than 20% at wavelengths of about 410 nm and 690 nm, and less than about 10% only at intermediate wavelengths. This reflectance is still too high for the structure to be used in applications. It should also be noted that the data were taken when light was directed to the glass surface of the multilayer structures at an angle of 20° relative to the normal of the sample. It is well known that the destructive interference has a strong dependence on the incident angle, and the reflection can be much higher when the incident angle is beyond 45°.